Regulating the fuel-to-air ratio of a carbureted mixture through a range of throttle settings is difficult, especially for engines equipped with non-metering, non-aspirating carburetors in which fuel is continuously sprayed under pressure into an air stream. Placing a pressure regulator in the fuel line suffers the disadvantage that the flow throttling passage constriction embodied in a pressure regulator impedes flow of fuel at full throttle settings and limits the maximum power which can be obtained from an engine.